This invention relates to swept impact seismic techniques (SIST) and apparatus for performing SIST.
High-resolution seismic surveys are carried out for locating and delineating ore and hydrocarbon deposits, for assessing the constructability ofrock and earth and for locating porous and possibly hydraulically conductive features with applications such as mining and exploration, rock engineering, monitoring of excavation works, disposal of hazardous waste.
The diversity of the applications of high-resolution seismics requires the data collection to be performed in very diverse conditions, e.g., in swamps, shallow water, on soil, gravel, pavement and rock, on tunnel walls and floors; in vertical, horizontal, up-going and down-going boreholes, drilled from the surface and from tunnels. The equipment must be able to operate in confined spaces and boreholes. The acquisition methods must be non-destructive and environmentally friendly. The while apparatus should be compact and mobile in construction and mining site conditions. Speed and routine are of the essence, to make the operation cost-effective. These are requirements for viable small-scale seismic investigation techniques.
A typical range for high-resolution surveys is hundreds of meters, which with favourable site conditions may be extended to 2 km. The minimum size of targets is of the order of meters, for localized anomalies, and fractions of a meter for laterally extensive features, e.g., fracture zones. To reach the desired resolution, small-scale seismic data must contain high frequencies of several hundreds Hertz (Hz) or more, which are usually associated with low-power sources. Conversely, the sources must deliver sufficient energy to carry the high frequencies through occasional highly attenuative media.
The high-frequency and high-energy requirements can be both fulfilled if the signal energy is built up over time, rather than being emitted as a short burst. The idea of injecting energy over a period of time is common to the technique known as Vibroseis (Crawford, J. M., Doty, W., and Lee, M. R., 1960. Continuous signal seismograph, Geophysics, 25, p.95-105), the Mini-Sosie (Barbier, M. B., Bondon, P., Mellinger, R., Viallix, J. R., 1976. Mini-SOSIE for land seismology. Geophysics Prosp., 24, p. 518-527) and the SIST (Park, C. B., Miller, R. D., Steeples, D. W. and Black, R. A., 1996, Swept Impact Seismic Technique (SIST). Geophysics, 61, no. 6, p.1789-1803).
The Swept Impact Seismic Technique (SIST) is a combination of the Vibroseis swept-frequency and the Mini-Sosie multi-impact ideas. With SIST, a low power impact source generates a series of seismic pulses, hence the relation to Mini-Sosie. However, instead of a pseudo random coding of the impact rates, a deterministic, monotonously varying rate is used, i.e., a swept impact rate, which makes SIST akin to Vibroseis. SIST is, reportedly, more time-efficient than Mini-Sosie. Compared to Vibroseis, with SIST a firm coupling to the rock or ground is not as critical. This is a clear advantage, as a firm, wide band contact is difficult to achieve in all situations, considering the diversity of experimental conditions encountered in small-scale surveys. Besides, the SIST apparatus is simpler and more portable than Vibroseis.
The SIST concept has previously been tested with shallow reflection applications, at low frequencies. We have now found SIST used with higher frequencies, to be a viable solution for high-resolution surveys, on ground surface, in underground openings and in boreholes, both methodologically and logistically.
According to the invention, a method of seismic investigation using the swept impact seismic technique comprises measuring the response of a site to be investigated to a series of controlled impact sequences applied at different points of the site, each sequence comprising impacts having a minimum repetition frequency in the range of 5 to 200 Hz and a maximum frequency having a ratio of at least 1.5 to 1 to the minimum frequency and the number of impacts in the sequence being at least 100.
The invention extends to apparatus for implementing such a method, comprising an electrically-actuated hammer having an impact frequency variable through a range of at least 1.5 to 1 within the range 10-500 Hz, and generating impacts characterized by substantial energy within the frequency range 100-5000 Hz, a drive circuit for energizing the hammer to deliver a sequence of at least 100 and preferably at least 500 impacts having a repetition frequency swept through a ratio of at least 1.5 to 1, and means for repositioning the hammer between sequences.
The apparatus may for example be configured to be applied to a surface of the site, which may be a ground surface or the wall of a tunnel, in which case the repetition frequency of the impacts is preferably in the range of 15-30 Hz., or within a borehole in which case the repetition frequency is preferably in the range 70-180 Hz.